k8s: update pod status logic to support native sidecars (#3169)

<!--- TITLE FORMAT: "component: short description", e.g. "k8s: add pod
log reader" -->

### Description
A k8s sidecar is an init container that starts before the main app
container and **continues to run**. This is unlike a regular init
container which always runs to completion before the app container
starts.

**Reading**
*
https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/753-sidecar-containers/README.md
* https://kubernetes.io/docs/concepts/workloads/pods/init-containers/
*
https://github.com/kubernetes/kubernetes/blob/66e34012255abf1bbd0956a712817dad77c69c41/pkg/printers/internalversion/printers.go#L905-L908

**PR changes**
we need to account for the difference between an init sidecar container
and a regular init container.
* to identify which is an init sidecar container, we check for where
restart policy is not nil and set to restartalways
* to identify if the init sidecar is ready, we check that it has a
running state and the container started is set to true. This is the same
pattern as upstream [kubectl get
pods](https://github.com/kubernetes/kubernetes/blob/66e34012255abf1bbd0956a712817dad77c69c41/pkg/printers/internalversion/printers.go#L905-L908)
command.


### Testing Performed
Existing unit tests
Manual

backend/service/k8s/pods.go

@@ -326,11 +326,22 @@ func getPodStatus(pod *corev1.Pod) string {
 		reason = pod.Status.Reason
 	}
 
+	nativeSidecarContainer := make(map[string]bool)
+	for _, container := range pod.Spec.InitContainers {
+		// https://kubernetes.io/docs/concepts/workloads/pods/sidecar-containers/
+		if container.RestartPolicy != nil && *container.RestartPolicy == corev1.ContainerRestartPolicyAlways {
+			nativeSidecarContainer[container.Name] = true
+		}
+	}
+
 	initializing := false
 	for i := range pod.Status.InitContainerStatuses {
 		container := pod.Status.InitContainerStatuses[i]
 		restarts += int(container.RestartCount)
 		switch {
+		case nativeSidecarContainer[container.Name] && container.Started != nil && *container.Started && container.Ready:
+			// https://github.com/kubernetes/kubernetes/blob/66e34012255abf1bbd0956a712817dad77c69c41/pkg/printers/internalversion/printers.go#L908
+			continue
 		case container.State.Terminated != nil && container.State.Terminated.ExitCode == 0:
 			continue
 		case container.State.Terminated != nil: